THE AZORES TRIPLE JUNCTION EVOLUTION SINCE 10-MA FROM AN AEROMAGNETIC SURVEY OF THE MID-ATLANTIC RIDGE

In the past two decades several models have been presented to describe the evolution and the present structure of the Azores Triple Junction. These models were mainly based on morphological analysis of sea bottom topography, sparse magnetic profiling, sidescan sonar surveying over the plateaus and global plate kinematic considerations for the North Atlantic. In this paper we follow a different approach: from a detailed aeromagnetic survey covering both sides of the Mid-Atlantic Ridge between 37-degrees-N and 40-degrees-30'N the magnetic anomalies up to anomaly 5 are accurately identified, allowing careful modelling of the kinematics of this region for the past 10 Ma and thus establishing a coherent framework for the design of geophysical models for the Azores Triple Junction. The analysis of magnetic anomalies and the use of Fourier domain inversion techniques show that the ridge is made up of six segments, each one varying in length from 50 to 60 km. The more continuous section of the ridge can be defined from the first four northern segments, although the North Azores Fracture Zone right-offsets the ridge at 39-degrees-30'N, 29-degrees-40'W. The fifth and sixth segments are, respectively, right-offset by the Acor Fracture Zone (at 38-degrees-23'N, 30-degrees-15'W) and by the Princess Alice Fracture Zone (at 38-degrees-00'N, 30-degrees-50'W). Anomaly identifications using a two-dimensional model and plate tectonics reconstruction techniques allowed the calculation of rotation pole parameters. The results thus obtained reveal that, at least between anomalies 5 and 3 (approximately 10-3.85 Ma), the Azores displayed an independent motion relative to the neighbouring plates and after anomaly 2A (2.45 Ma) the Azores moved attached to the Eurasian plate. The triple junction (Azores-North America-Africa, or Eurasia-North America-Africa) moved northward from approximately 38-degrees-00'N, 30-degrees-50'W to approximately 38-degrees-20'N, 30-degrees-15'W (between 4 and 3A) and probably to 38-degrees-50'N, 30-degrees-00'W at anomaly 2A time. A detailed reconstruction model is presented.